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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="drd-manual"></a>8. DRD: a thread error detector</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="sect1"><a href="drd-manual.html#drd-manual.overview">8.1. Overview</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.mt-progr-models">8.1.1. Multithreaded Programming Paradigms</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.pthreads-model">8.1.2. POSIX Threads Programming Model</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.mt-problems">8.1.3. Multithreaded Programming Problems</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.data-race-detection">8.1.4. Data Race Detection</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="drd-manual.html#drd-manual.using-drd">8.2. Using DRD</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.options">8.2.1. DRD Command-line Options</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.data-races">8.2.2. Detected Errors: Data Races</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.lock-contention">8.2.3. Detected Errors: Lock Contention</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.api-checks">8.2.4. Detected Errors: Misuse of the POSIX threads API</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.clientreqs">8.2.5. Client Requests</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.CXX11">8.2.6. Debugging C++11 Programs</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.gnome">8.2.7. Debugging GNOME Programs</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.boost.thread">8.2.8. Debugging Boost.Thread Programs</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.openmp">8.2.9. Debugging OpenMP Programs</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.cust-mem-alloc">8.2.10. DRD and Custom Memory Allocators</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.drd-versus-memcheck">8.2.11. DRD Versus Memcheck</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.resource-requirements">8.2.12. Resource Requirements</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.effective-use">8.2.13. Hints and Tips for Effective Use of DRD</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="drd-manual.html#drd-manual.Pthreads">8.3. Using the POSIX Threads API Effectively</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.mutex-types">8.3.1. Mutex types</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.condvar">8.3.2. Condition variables</a></span></dt>
<dt><span class="sect2"><a href="drd-manual.html#drd-manual.pctw">8.3.3. pthread_cond_timedwait and timeouts</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="drd-manual.html#drd-manual.limitations">8.4. Limitations</a></span></dt>
<dt><span class="sect1"><a href="drd-manual.html#drd-manual.feedback">8.5. Feedback</a></span></dt>
</dl>
</div>
<p>To use this tool, you must specify
<code class="option">--tool=drd</code>
on the Valgrind command line.</p>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="drd-manual.overview"></a>8.1. Overview</h2></div></div></div>
<p>
DRD is a Valgrind tool for detecting errors in multithreaded C and C++
programs. The tool works for any program that uses the POSIX threading
primitives or that uses threading concepts built on top of the POSIX threading
primitives.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.mt-progr-models"></a>8.1.1. Multithreaded Programming Paradigms</h3></div></div></div>
<p>
There are two possible reasons for using multithreading in a program:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      To model concurrent activities. Assigning one thread to each activity
      can be a great simplification compared to multiplexing the states of
      multiple activities in a single thread. This is why most server software
      and embedded software is multithreaded.
    </p></li>
<li class="listitem"><p>
      To use multiple CPU cores simultaneously for speeding up
      computations. This is why many High Performance Computing (HPC)
      applications are multithreaded.
    </p></li>
</ul></div>
<p>
</p>
<p>
Multithreaded programs can use one or more of the following programming
paradigms. Which paradigm is appropriate depends e.g. on the application type.
Some examples of multithreaded programming paradigms are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      Locking. Data that is shared over threads is protected from concurrent
      accesses via locking. E.g. the POSIX threads library, the Qt library
      and the Boost.Thread library support this paradigm directly.
    </p></li>
<li class="listitem"><p>
      Message passing. No data is shared between threads, but threads exchange
      data by passing messages to each other. Examples of implementations of
      the message passing paradigm are MPI and CORBA.
    </p></li>
<li class="listitem"><p>
      Automatic parallelization. A compiler converts a sequential program into
      a multithreaded program. The original program may or may not contain
      parallelization hints. One example of such parallelization hints is the
      OpenMP standard. In this standard a set of directives are defined which
      tell a compiler how to parallelize a C, C++ or Fortran program. OpenMP
      is well suited for computational intensive applications. As an example,
      an open source image processing software package is using OpenMP to
      maximize performance on systems with multiple CPU
      cores. GCC supports the
      OpenMP standard from version 4.2.0 on.
    </p></li>
<li class="listitem"><p>
      Software Transactional Memory (STM). Any data that is shared between
      threads is updated via transactions. After each transaction it is
      verified whether there were any conflicting transactions. If there were
      conflicts, the transaction is aborted, otherwise it is committed. This
      is a so-called optimistic approach. There is a prototype of the Intel C++
      Compiler available that supports STM. Research about the addition of
      STM support to GCC is ongoing.
    </p></li>
</ul></div>
<p>
</p>
<p>
DRD supports any combination of multithreaded programming paradigms as
long as the implementation of these paradigms is based on the POSIX
threads primitives. DRD however does not support programs that use
e.g. Linux' futexes directly. Attempts to analyze such programs with
DRD will cause DRD to report many false positives.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.pthreads-model"></a>8.1.2. POSIX Threads Programming Model</h3></div></div></div>
<p>
POSIX threads, also known as Pthreads, is the most widely available
threading library on Unix systems.
</p>
<p>
The POSIX threads programming model is based on the following abstractions:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      A shared address space. All threads running within the same
      process share the same address space. All data, whether shared or
      not, is identified by its address.
    </p></li>
<li class="listitem"><p>
      Regular load and store operations, which allow to read values
      from or to write values to the memory shared by all threads
      running in the same process.
    </p></li>
<li class="listitem"><p>
      Atomic store and load-modify-store operations. While these are
      not mentioned in the POSIX threads standard, most
      microprocessors support atomic memory operations.
    </p></li>
<li class="listitem"><p>
      Threads. Each thread represents a concurrent activity.
    </p></li>
<li class="listitem"><p>
      Synchronization objects and operations on these synchronization
      objects. The following types of synchronization objects have been
      defined in the POSIX threads standard: mutexes, condition variables,
      semaphores, reader-writer synchronization objects, barriers and
      spinlocks.
    </p></li>
</ul></div>
<p>
</p>
<p>
Which source code statements generate which memory accesses depends on
the <span class="emphasis"><em>memory model</em></span> of the programming language being
used. There is not yet a definitive memory model for the C and C++
languages. For a draft memory model, see also the document
<a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html" target="_top">
WG21/N2338: Concurrency memory model compiler consequences</a>.
</p>
<p>
For more information about POSIX threads, see also the Single UNIX
Specification version 3, also known as
<a class="ulink" href="http://www.opengroup.org/onlinepubs/000095399/idx/threads.html" target="_top">
IEEE Std 1003.1</a>.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.mt-problems"></a>8.1.3. Multithreaded Programming Problems</h3></div></div></div>
<p>
Depending on which multithreading paradigm is being used in a program,
one or more of the following problems can occur:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      Data races. One or more threads access the same memory location without
      sufficient locking. Most but not all data races are programming errors
      and are the cause of subtle and hard-to-find bugs.
    </p></li>
<li class="listitem"><p>
      Lock contention. One thread blocks the progress of one or more other
      threads by holding a lock too long.
    </p></li>
<li class="listitem"><p>
      Improper use of the POSIX threads API. Most implementations of the POSIX
      threads API have been optimized for runtime speed. Such implementations
      will not complain on certain errors, e.g. when a mutex is being unlocked
      by another thread than the thread that obtained a lock on the mutex.
    </p></li>
<li class="listitem"><p>
      Deadlock. A deadlock occurs when two or more threads wait for
      each other indefinitely.
    </p></li>
<li class="listitem"><p>
      False sharing. If threads that run on different processor cores
      access different variables located in the same cache line
      frequently, this will slow down the involved threads a lot due
      to frequent exchange of cache lines.
    </p></li>
</ul></div>
<p>
</p>
<p>
Although the likelihood of the occurrence of data races can be reduced
through a disciplined programming style, a tool for automatic
detection of data races is a necessity when developing multithreaded
software. DRD can detect these, as well as lock contention and
improper use of the POSIX threads API.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.data-race-detection"></a>8.1.4. Data Race Detection</h3></div></div></div>
<p>
The result of load and store operations performed by a multithreaded program
depends on the order in which memory operations are performed. This order is
determined by:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem"><p>
      All memory operations performed by the same thread are performed in
      <span class="emphasis"><em>program order</em></span>, that is, the order determined by the
      program source code and the results of previous load operations.
    </p></li>
<li class="listitem"><p>
      Synchronization operations determine certain ordering constraints on
      memory operations performed by different threads. These ordering
      constraints are called the <span class="emphasis"><em>synchronization order</em></span>.
    </p></li>
</ol></div>
<p>
The combination of program order and synchronization order is called the
<span class="emphasis"><em>happens-before relationship</em></span>. This concept was first
defined by S. Adve et al in the paper <span class="emphasis"><em>Detecting data races on weak
memory systems</em></span>, ACM SIGARCH Computer Architecture News, v.19 n.3,
p.234-243, May 1991.
</p>
<p>
Two memory operations <span class="emphasis"><em>conflict</em></span> if both operations are
performed by different threads, refer to the same memory location and at least
one of them is a store operation.
</p>
<p>
A multithreaded program is <span class="emphasis"><em>data-race free</em></span> if all
conflicting memory accesses are ordered by synchronization
operations.
</p>
<p>
A well known way to ensure that a multithreaded program is data-race
free is to ensure that a locking discipline is followed. It is e.g.
possible to associate a mutex with each shared data item, and to hold
a lock on the associated mutex while the shared data is accessed.
</p>
<p>
All programs that follow a locking discipline are data-race free, but not all
data-race free programs follow a locking discipline. There exist multithreaded
programs where access to shared data is arbitrated via condition variables,
semaphores or barriers. As an example, a certain class of HPC applications
consists of a sequence of computation steps separated in time by barriers, and
where these barriers are the only means of synchronization. Although there are
many conflicting memory accesses in such applications and although such
applications do not make use mutexes, most of these applications do not
contain data races.
</p>
<p>
There exist two different approaches for verifying the correctness of
multithreaded programs at runtime. The approach of the so-called Eraser
algorithm is to verify whether all shared memory accesses follow a consistent
locking strategy. And the happens-before data race detectors verify directly
whether all interthread memory accesses are ordered by synchronization
operations. While the last approach is more complex to implement, and while it
is more sensitive to OS scheduling, it is a general approach that works for
all classes of multithreaded programs. An important advantage of
happens-before data race detectors is that these do not report any false
positives.
</p>
<p>
DRD is based on the happens-before algorithm.
</p>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="drd-manual.using-drd"></a>8.2. Using DRD</h2></div></div></div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.options"></a>8.2.1. DRD Command-line Options</h3></div></div></div>
<p>The following command-line options are available for controlling the
behavior of the DRD tool itself:</p>
<div class="variablelist">
<a name="drd.opts.list"></a><dl class="variablelist">
<dt><span class="term">
      <code class="option">--check-stack-var=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Controls whether DRD detects data races on stack
        variables. Verifying stack variables is disabled by default because
        most programs do not share stack variables over threads.
      </p></dd>
<dt><span class="term">
      <code class="option">--exclusive-threshold=&lt;n&gt; [default: off]</code>
    </span></dt>
<dd><p>
        Print an error message if any mutex or writer lock has been
        held longer than the time specified in milliseconds. This
        option enables the detection of lock contention.
      </p></dd>
<dt><span class="term">
      <code class="option">--join-list-vol=&lt;n&gt; [default: 10]</code>
    </span></dt>
<dd><p>
        Data races that occur between a statement at the end of one thread
	and another thread can be missed if memory access information is
	discarded immediately after a thread has been joined. This option
	allows one to specify for how many joined threads memory access information
	should be retained.
      </p></dd>
<dt><span class="term">
      <code class="option">
        --first-race-only=&lt;yes|no&gt; [default: no]
      </code>
    </span></dt>
<dd><p>
        Whether to report only the first data race that has been detected on a
        memory location or all data races that have been detected on a memory
        location.
      </p></dd>
<dt><span class="term">
      <code class="option">
        --free-is-write=&lt;yes|no&gt; [default: no]
      </code>
    </span></dt>
<dd>
<p>
        Whether to report races between accessing memory and freeing
        memory. Enabling this option may cause DRD to run slightly
        slower. Notes:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
	    Don't enable this option when using custom memory allocators
	    that use
	    the <code class="computeroutput">VG_USERREQ__MALLOCLIKE_BLOCK</code>
	    and <code class="computeroutput">VG_USERREQ__FREELIKE_BLOCK</code>
	    because that would result in false positives.
	  </p></li>
<li class="listitem"><p>Don't enable this option when using reference-counted
	    objects because that will result in false positives, even when
	    that code has been annotated properly with
	    <code class="computeroutput">ANNOTATE_HAPPENS_BEFORE</code>
	    and <code class="computeroutput">ANNOTATE_HAPPENS_AFTER</code>. See
	    e.g.  the output of the following command for an example:
	    <code class="computeroutput">valgrind --tool=drd --free-is-write=yes
	      drd/tests/annotate_smart_pointer</code>.
	  </p></li>
</ul></div>
</dd>
<dt><span class="term">
      <code class="option">
        --report-signal-unlocked=&lt;yes|no&gt; [default: yes]
      </code>
    </span></dt>
<dd><p>
        Whether to report calls to
        <code class="function">pthread_cond_signal</code> and
        <code class="function">pthread_cond_broadcast</code> where the mutex
        associated with the signal through
        <code class="function">pthread_cond_wait</code> or
        <code class="function">pthread_cond_timed_wait</code>is not locked at
        the time the signal is sent.  Sending a signal without holding
        a lock on the associated mutex is a common programming error
        which can cause subtle race conditions and unpredictable
        behavior. There exist some uncommon synchronization patterns
        however where it is safe to send a signal without holding a
        lock on the associated mutex.
      </p></dd>
<dt><span class="term">
      <code class="option">--segment-merging=&lt;yes|no&gt; [default: yes]</code>
    </span></dt>
<dd><p>
        Controls segment merging. Segment merging is an algorithm to
        limit memory usage of the data race detection
        algorithm. Disabling segment merging may improve the accuracy
        of the so-called 'other segments' displayed in race reports
        but can also trigger an out of memory error.
      </p></dd>
<dt><span class="term">
      <code class="option">--segment-merging-interval=&lt;n&gt; [default: 10]</code>
    </span></dt>
<dd><p>
        Perform segment merging only after the specified number of new
        segments have been created. This is an advanced configuration option
        that allows one to choose whether to minimize DRD's memory usage by
        choosing a low value or to let DRD run faster by choosing a slightly
        higher value. The optimal value for this parameter depends on the
        program being analyzed. The default value works well for most programs.
      </p></dd>
<dt><span class="term">
      <code class="option">--shared-threshold=&lt;n&gt; [default: off]</code>
    </span></dt>
<dd><p>
        Print an error message if a reader lock has been held longer
        than the specified time (in milliseconds). This option enables
        the detection of lock contention.
      </p></dd>
<dt><span class="term">
      <code class="option">--show-confl-seg=&lt;yes|no&gt; [default: yes]</code>
    </span></dt>
<dd><p>
         Show conflicting segments in race reports. Since this
         information can help to find the cause of a data race, this
         option is enabled by default. Disabling this option makes the
         output of DRD more compact.
      </p></dd>
<dt><span class="term">
      <code class="option">--show-stack-usage=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Print stack usage at thread exit time. When a program creates a large
        number of threads it becomes important to limit the amount of virtual
        memory allocated for thread stacks. This option makes it possible to
        observe how much stack memory has been used by each thread of the
        client program. Note: the DRD tool itself allocates some temporary
        data on the client thread stack. The space necessary for this
        temporary data must be allocated by the client program when it
        allocates stack memory, but is not included in stack usage reported by
        DRD.
      </p></dd>
<dt><span class="term">
      <code class="option">--ignore-thread-creation=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd>
<p>
        Controls whether all activities during thread creation should be
        ignored. By default enabled only on Solaris.
        Solaris provides higher throughput, parallelism and scalability than
        other operating systems, at the cost of more fine-grained locking
        activity. This means for example that when a thread is created under
        glibc, just one big lock is used for all thread setup. Solaris libc
        uses several fine-grained locks and the creator thread resumes its
        activities as soon as possible, leaving for example stack and TLS setup
        sequence to the created thread.
        This situation confuses DRD as it assumes there is some false ordering
        in place between creator and created thread; and therefore many types
        of race conditions in the application would not be reported. To prevent
        such false ordering, this command line option is set to
        <code class="computeroutput">yes</code> by default on Solaris.
        All activity (loads, stores, client requests) is therefore ignored
        during:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
	    pthread_create() call in the creator thread
	  </p></li>
<li class="listitem"><p>
	    thread creation phase (stack and TLS setup) in the created thread
	  </p></li>
</ul></div>
</dd>
</dl>
</div>
<p>
The following options are available for monitoring the behavior of the
client program:
</p>
<div class="variablelist">
<a name="drd.debugopts.list"></a><dl class="variablelist">
<dt><span class="term">
      <code class="option">--trace-addr=&lt;address&gt; [default: none]</code>
    </span></dt>
<dd><p>
        Trace all load and store activity for the specified
        address. This option may be specified more than once.
      </p></dd>
<dt><span class="term">
      <code class="option">--ptrace-addr=&lt;address&gt; [default: none]</code>
    </span></dt>
<dd><p>
        Trace all load and store activity for the specified address and keep
        doing that even after the memory at that address has been freed and
        reallocated.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-alloc=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all memory allocations and deallocations. May produce a huge
        amount of output.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-barrier=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all barrier activity.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-cond=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all condition variable activity.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-fork-join=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all thread creation and all thread termination events.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-hb=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace execution of the <code class="literal">ANNOTATE_HAPPENS_BEFORE()</code>,
	<code class="literal">ANNOTATE_HAPPENS_AFTER()</code> and
	<code class="literal">ANNOTATE_HAPPENS_DONE()</code> client requests.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-mutex=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all mutex activity.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-rwlock=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
         Trace all reader-writer lock activity.
      </p></dd>
<dt><span class="term">
      <code class="option">--trace-semaphore=&lt;yes|no&gt; [default: no]</code>
    </span></dt>
<dd><p>
        Trace all semaphore activity.
      </p></dd>
</dl>
</div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.data-races"></a>8.2.2. Detected Errors: Data Races</h3></div></div></div>
<p>
DRD prints a message every time it detects a data race. Please keep
the following in mind when interpreting DRD's output:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      Every thread is assigned a <span class="emphasis"><em>thread ID</em></span> by the DRD
      tool. A thread ID is a number. Thread ID's start at one and are never
      recycled.
    </p></li>
<li class="listitem"><p>
      The term <span class="emphasis"><em>segment</em></span> refers to a consecutive
      sequence of load, store and synchronization operations, all
      issued by the same thread. A segment always starts and ends at a
      synchronization operation. Data race analysis is performed
      between segments instead of between individual load and store
      operations because of performance reasons.
    </p></li>
<li class="listitem"><p>
      There are always at least two memory accesses involved in a data
      race. Memory accesses involved in a data race are called
      <span class="emphasis"><em>conflicting memory accesses</em></span>. DRD prints a
      report for each memory access that conflicts with a past memory
      access.
    </p></li>
</ul></div>
<p>
</p>
<p>
Below you can find an example of a message printed by DRD when it
detects a data race:
</p>
<pre class="programlisting">
$ valgrind --tool=drd --read-var-info=yes drd/tests/rwlock_race
...
==9466== Thread 3:
==9466== Conflicting load by thread 3 at 0x006020b8 size 4
==9466==    at 0x400B6C: thread_func (rwlock_race.c:29)
==9466==    by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466==    by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466==    by 0x53250CC: clone (in /lib64/libc-2.8.so)
==9466== Location 0x6020b8 is 0 bytes inside local var "s_racy"
==9466== declared at rwlock_race.c:18, in frame #0 of thread 3
==9466== Other segment start (thread 2)
==9466==    at 0x4C2847D: pthread_rwlock_rdlock* (drd_pthread_intercepts.c:813)
==9466==    by 0x400B6B: thread_func (rwlock_race.c:28)
==9466==    by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466==    by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466==    by 0x53250CC: clone (in /lib64/libc-2.8.so)
==9466== Other segment end (thread 2)
==9466==    at 0x4C28B54: pthread_rwlock_unlock* (drd_pthread_intercepts.c:912)
==9466==    by 0x400B84: thread_func (rwlock_race.c:30)
==9466==    by 0x4C291DF: vg_thread_wrapper (drd_pthread_intercepts.c:186)
==9466==    by 0x4E3403F: start_thread (in /lib64/libpthread-2.8.so)
==9466==    by 0x53250CC: clone (in /lib64/libc-2.8.so)
...
</pre>
<p>
The above report has the following meaning:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      The number in the column on the left is the process ID of the
      process being analyzed by DRD.
    </p></li>
<li class="listitem"><p>
      The first line ("Thread 3") tells you the thread ID for
      the thread in which context the data race has been detected.
    </p></li>
<li class="listitem"><p>
      The next line tells which kind of operation was performed (load or
      store) and by which thread. On the same line the start address and the
      number of bytes involved in the conflicting access are also displayed.
    </p></li>
<li class="listitem"><p>
      Next, the call stack of the conflicting access is displayed. If
      your program has been compiled with debug information
      (<code class="option">-g</code>), this call stack will include file names and
      line numbers. The two
      bottommost frames in this call stack (<code class="function">clone</code>
      and <code class="function">start_thread</code>) show how the NPTL starts
      a thread. The third frame
      (<code class="function">vg_thread_wrapper</code>) is added by DRD. The
      fourth frame (<code class="function">thread_func</code>) is the first
      interesting line because it shows the thread entry point, that
      is the function that has been passed as the third argument to
      <code class="function">pthread_create</code>.
    </p></li>
<li class="listitem"><p>
      Next, the allocation context for the conflicting address is
      displayed. For dynamically allocated data the allocation call
      stack is shown. For static variables and stack variables the
      allocation context is only shown when the option
      <code class="option">--read-var-info=yes</code> has been
      specified. Otherwise DRD will print <code class="computeroutput">Allocation
      context: unknown</code>.
    </p></li>
<li class="listitem">
<p>
      A conflicting access involves at least two memory accesses. For
      one of these accesses an exact call stack is displayed, and for
      the other accesses an approximate call stack is displayed,
      namely the start and the end of the segments of the other
      accesses. This information can be interpreted as follows:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem"><p>
            Start at the bottom of both call stacks, and count the
            number stack frames with identical function name, file
            name and line number. In the above example the three
            bottommost frames are identical
            (<code class="function">clone</code>,
            <code class="function">start_thread</code> and
            <code class="function">vg_thread_wrapper</code>).
          </p></li>
<li class="listitem"><p>
            The next higher stack frame in both call stacks now tells
            you between in which source code region the other memory
            access happened. The above output tells that the other
            memory access involved in the data race happened between
            source code lines 28 and 30 in file
            <code class="computeroutput">rwlock_race.c</code>.
          </p></li>
</ol></div>
<p>
    </p>
</li>
</ul></div>
<p>
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.lock-contention"></a>8.2.3. Detected Errors: Lock Contention</h3></div></div></div>
<p>
Threads must be able to make progress without being blocked for too long by
other threads. Sometimes a thread has to wait until a mutex or reader-writer
synchronization object is unlocked by another thread. This is called
<span class="emphasis"><em>lock contention</em></span>.
</p>
<p>
Lock contention causes delays. Such delays should be as short as
possible. The two command line options
<code class="literal">--exclusive-threshold=&lt;n&gt;</code> and
<code class="literal">--shared-threshold=&lt;n&gt;</code> make it possible to
detect excessive lock contention by making DRD report any lock that
has been held longer than the specified threshold. An example:
</p>
<pre class="programlisting">
$ valgrind --tool=drd --exclusive-threshold=10 drd/tests/hold_lock -i 500
...
==10668== Acquired at:
==10668==    at 0x4C267C8: pthread_mutex_lock (drd_pthread_intercepts.c:395)
==10668==    by 0x400D92: main (hold_lock.c:51)
==10668== Lock on mutex 0x7fefffd50 was held during 503 ms (threshold: 10 ms).
==10668==    at 0x4C26ADA: pthread_mutex_unlock (drd_pthread_intercepts.c:441)
==10668==    by 0x400DB5: main (hold_lock.c:55)
...
</pre>
<p>
The <code class="literal">hold_lock</code> test program holds a lock as long as
specified by the <code class="literal">-i</code> (interval) argument. The DRD
output reports that the lock acquired at line 51 in source file
<code class="literal">hold_lock.c</code> and released at line 55 was held during
503 ms, while a threshold of 10 ms was specified to DRD.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.api-checks"></a>8.2.4. Detected Errors: Misuse of the POSIX threads API</h3></div></div></div>
<p>
  DRD is able to detect and report the following misuses of the POSIX
  threads API:
  </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
        Passing the address of one type of synchronization object
        (e.g. a mutex) to a POSIX API call that expects a pointer to
        another type of synchronization object (e.g. a condition
        variable).
      </p></li>
<li class="listitem"><p>
        Attempts to unlock a mutex that has not been locked.
      </p></li>
<li class="listitem"><p>
        Attempts to unlock a mutex that was locked by another thread.
      </p></li>
<li class="listitem"><p>
        Attempts to lock a mutex of type
        <code class="literal">PTHREAD_MUTEX_NORMAL</code> or a spinlock
        recursively.
      </p></li>
<li class="listitem"><p>
        Destruction or deallocation of a locked mutex.
      </p></li>
<li class="listitem"><p>
        Sending a signal to a condition variable while no lock is held
        on the mutex associated with the condition variable.
      </p></li>
<li class="listitem"><p>
        Calling <code class="function">pthread_cond_wait</code> on a mutex
        that is not locked, that is locked by another thread or that
        has been locked recursively.
      </p></li>
<li class="listitem"><p>
        Associating two different mutexes with a condition variable
        through <code class="function">pthread_cond_wait</code>.
      </p></li>
<li class="listitem"><p>
        Destruction or deallocation of a condition variable that is
        being waited upon.
      </p></li>
<li class="listitem"><p>
        Destruction or deallocation of a locked reader-writer synchronization
        object.
      </p></li>
<li class="listitem"><p>
        Attempts to unlock a reader-writer synchronization object that was not
        locked by the calling thread.
      </p></li>
<li class="listitem"><p>
        Attempts to recursively lock a reader-writer synchronization object
        exclusively.
      </p></li>
<li class="listitem"><p>
        Attempts to pass the address of a user-defined reader-writer
        synchronization object to a POSIX threads function.
      </p></li>
<li class="listitem"><p>
        Attempts to pass the address of a POSIX reader-writer synchronization
        object to one of the annotations for user-defined reader-writer
        synchronization objects.
      </p></li>
<li class="listitem"><p>
        Reinitialization of a mutex, condition variable, reader-writer
        lock, semaphore or barrier.
      </p></li>
<li class="listitem"><p>
        Destruction or deallocation of a semaphore or barrier that is
        being waited upon.
      </p></li>
<li class="listitem"><p>
        Missing synchronization between barrier wait and barrier destruction.
      </p></li>
<li class="listitem"><p>
        Exiting a thread without first unlocking the spinlocks, mutexes or
        reader-writer synchronization objects that were locked by that thread.
      </p></li>
<li class="listitem"><p>
        Passing an invalid thread ID to <code class="function">pthread_join</code>
        or <code class="function">pthread_cancel</code>.
      </p></li>
</ul></div>
<p>
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.clientreqs"></a>8.2.5. Client Requests</h3></div></div></div>
<p>
Just as for other Valgrind tools it is possible to let a client program
interact with the DRD tool through client requests. In addition to the
client requests several macros have been defined that allow to use the
client requests in a convenient way.
</p>
<p>
The interface between client programs and the DRD tool is defined in
the header file <code class="literal">&lt;valgrind/drd.h&gt;</code>. The
available macros and client requests are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      The macro <code class="literal">DRD_GET_VALGRIND_THREADID</code> and the
      corresponding client
      request <code class="varname">VG_USERREQ__DRD_GET_VALGRIND_THREAD_ID</code>.
      Query the thread ID that has been assigned by the Valgrind core to the
      thread executing this client request. Valgrind's thread ID's start at
      one and are recycled in case a thread stops.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">DRD_GET_DRD_THREADID</code> and the corresponding
      client request <code class="varname">VG_USERREQ__DRD_GET_DRD_THREAD_ID</code>.
      Query the thread ID that has been assigned by DRD to the thread
      executing this client request. These are the thread ID's reported by DRD
      in data race reports and in trace messages. DRD's thread ID's start at
      one and are never recycled.
    </p></li>
<li class="listitem"><p>
      The macros <code class="literal">DRD_IGNORE_VAR(x)</code>,
      <code class="literal">ANNOTATE_TRACE_MEMORY(&amp;x)</code> and the corresponding
      client request <code class="varname">VG_USERREQ__DRD_START_SUPPRESSION</code>. Some
      applications contain intentional races. There exist e.g. applications
      where the same value is assigned to a shared variable from two different
      threads. It may be more convenient to suppress such races than to solve
      these. This client request allows one to suppress such races.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">DRD_STOP_IGNORING_VAR(x)</code> and the
      corresponding client request
      <code class="varname">VG_USERREQ__DRD_FINISH_SUPPRESSION</code>. Tell DRD
      to no longer ignore data races for the address range that was suppressed
      either via the macro <code class="literal">DRD_IGNORE_VAR(x)</code> or via the
      client request <code class="varname">VG_USERREQ__DRD_START_SUPPRESSION</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">DRD_TRACE_VAR(x)</code>. Trace all load and store
      activity for the address range starting at <code class="literal">&amp;x</code> and
      occupying <code class="literal">sizeof(x)</code> bytes. When DRD reports a data
      race on a specified variable, and it's not immediately clear which
      source code statements triggered the conflicting accesses, it can be
      very helpful to trace all activity on the offending memory location.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">DRD_STOP_TRACING_VAR(x)</code>. Stop tracing load
      and store activity for the address range starting
      at <code class="literal">&amp;x</code> and occupying <code class="literal">sizeof(x)</code>
      bytes.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_TRACE_MEMORY(&amp;x)</code>. Trace all
      load and store activity that touches at least the single byte at the
      address <code class="literal">&amp;x</code>.
    </p></li>
<li class="listitem"><p>
      The client request <code class="varname">VG_USERREQ__DRD_START_TRACE_ADDR</code>,
      which allows one to trace all load and store activity for the specified
      address range.
    </p></li>
<li class="listitem"><p>
      The client
      request <code class="varname">VG_USERREQ__DRD_STOP_TRACE_ADDR</code>. Do no longer
      trace load and store activity for the specified address range.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_HAPPENS_BEFORE(addr)</code> tells DRD to
      insert a mark. Insert this macro just after an access to the variable at
      the specified address has been performed.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_HAPPENS_AFTER(addr)</code> tells DRD that
      the next access to the variable at the specified address should be
      considered to have happened after the access just before the latest
      <code class="literal">ANNOTATE_HAPPENS_BEFORE(addr)</code> annotation that
      references the same variable. The purpose of these two macros is to tell
      DRD about the order of inter-thread memory accesses implemented via
      atomic memory operations. See
      also <code class="literal">drd/tests/annotate_smart_pointer.cpp</code> for an
      example.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_RWLOCK_CREATE(rwlock)</code> tells DRD
      that the object at address <code class="literal">rwlock</code> is a
      reader-writer synchronization object that is not a
      <code class="literal">pthread_rwlock_t</code> synchronization object.  See
      also <code class="literal">drd/tests/annotate_rwlock.c</code> for an example.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_RWLOCK_DESTROY(rwlock)</code> tells DRD
      that the reader-writer synchronization object at
      address <code class="literal">rwlock</code> has been destroyed.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_WRITERLOCK_ACQUIRED(rwlock)</code> tells
      DRD that a writer lock has been acquired on the reader-writer
      synchronization object at address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_READERLOCK_ACQUIRED(rwlock)</code> tells
      DRD that a reader lock has been acquired on the reader-writer
      synchronization object at address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_RWLOCK_ACQUIRED(rwlock, is_w)</code>
      tells DRD that a writer lock (when <code class="literal">is_w != 0</code>) or that
      a reader lock (when <code class="literal">is_w == 0</code>) has been acquired on
      the reader-writer synchronization object at
      address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_WRITERLOCK_RELEASED(rwlock)</code> tells
      DRD that a writer lock has been released on the reader-writer
      synchronization object at address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_READERLOCK_RELEASED(rwlock)</code> tells
      DRD that a reader lock has been released on the reader-writer
      synchronization object at address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_RWLOCK_RELEASED(rwlock, is_w)</code>
      tells DRD that a writer lock (when <code class="literal">is_w != 0</code>) or that
      a reader lock (when <code class="literal">is_w == 0</code>) has been released on
      the reader-writer synchronization object at
      address <code class="literal">rwlock</code>.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BARRIER_INIT(barrier, count,
      reinitialization_allowed)</code> tells DRD that a new barrier object
      at the address <code class="literal">barrier</code> has been initialized,
      that <code class="literal">count</code> threads participate in each barrier and
      also whether or not barrier reinitialization without intervening
      destruction should be reported as an error. See
      also <code class="literal">drd/tests/annotate_barrier.c</code> for an example.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BARRIER_DESTROY(barrier)</code>
      tells DRD that a barrier object is about to be destroyed.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BARRIER_WAIT_BEFORE(barrier)</code>
      tells DRD that waiting for a barrier will start.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BARRIER_WAIT_AFTER(barrier)</code>
      tells DRD that waiting for a barrier has finished.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BENIGN_RACE_SIZED(addr, size,
      descr)</code> tells DRD that any races detected on the specified
      address are benign and hence should not be
      reported. The <code class="literal">descr</code> argument is ignored but can be
      used to document why data races on <code class="literal">addr</code> are benign.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_BENIGN_RACE_STATIC(var, descr)</code>
      tells DRD that any races detected on the specified static variable are
      benign and hence should not be reported. The <code class="literal">descr</code>
      argument is ignored but can be used to document why data races
      on <code class="literal">var</code> are benign. Note: this macro can only be
      used in C++ programs and not in C programs.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_READS_BEGIN</code> tells
      DRD to ignore all memory loads performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_READS_END</code> tells
      DRD to stop ignoring the memory loads performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_WRITES_BEGIN</code> tells
      DRD to ignore all memory stores performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_WRITES_END</code> tells
      DRD to stop ignoring the memory stores performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN</code> tells
      DRD to ignore all memory accesses performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_IGNORE_READS_AND_WRITES_END</code> tells
      DRD to stop ignoring the memory accesses performed by the current thread.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_NEW_MEMORY(addr, size)</code> tells
      DRD that the specified memory range has been allocated by a custom
      memory allocator in the client program and that the client program
      will start using this memory range.
    </p></li>
<li class="listitem"><p>
      The macro <code class="literal">ANNOTATE_THREAD_NAME(name)</code> tells DRD to
      associate the specified name with the current thread and to include this
      name in the error messages printed by DRD.
    </p></li>
<li class="listitem"><p>
      The macros <code class="literal">VALGRIND_MALLOCLIKE_BLOCK</code> and
      <code class="literal">VALGRIND_FREELIKE_BLOCK</code> from the Valgrind core are
      implemented;  they are described in 
      <a class="xref" href="manual-core-adv.html#manual-core-adv.clientreq" title="3.1. The Client Request mechanism">The Client Request mechanism</a>.
    </p></li>
</ul></div>
<p>
</p>
<p>
Note: if you compiled Valgrind yourself, the header file
<code class="literal">&lt;valgrind/drd.h&gt;</code> will have been installed in
the directory <code class="literal">/usr/include</code> by the command
<code class="literal">make install</code>. If you obtained Valgrind by
installing it as a package however, you will probably have to install
another package with a name like <code class="literal">valgrind-devel</code>
before Valgrind's header files are available.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.CXX11"></a>8.2.6. Debugging C++11 Programs</h3></div></div></div>
<p>If you want to use the C++11 class std::thread you will need to do the
  following to annotate the std::shared_ptr&lt;&gt; objects used in the
  implementation of that class:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>Add the following code at the start of a common header or at the
    start of each source file, before any C++ header files are included:</p>
<pre class="programlisting">
#include &lt;valgrind/drd.h&gt;
#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(addr) ANNOTATE_HAPPENS_BEFORE(addr)
#define _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(addr)  ANNOTATE_HAPPENS_AFTER(addr)
</pre>
</li>
<li class="listitem"><p>Download the gcc source code and from source file
    libstdc++-v3/src/c++11/thread.cc copy the implementation of the
    <code class="computeroutput">execute_native_thread_routine()</code>
    and <code class="computeroutput">std::thread::_M_start_thread()</code>
    functions into a source file that is linked with your application. Make
    sure that also in this source file the
    _GLIBCXX_SYNCHRONIZATION_HAPPENS_*() macros are defined properly.</p></li>
</ul></div>
<p>
</p>
<p>For more information, see also <span class="emphasis"><em>The
GNU C++ Library Manual, Debugging Support</em></span>
(<a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html" target="_top">http://gcc.gnu.org/onlinedocs/libstdc++/manual/debug.html</a>).</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.gnome"></a>8.2.7. Debugging GNOME Programs</h3></div></div></div>
<p>
GNOME applications use the threading primitives provided by the
<code class="computeroutput">glib</code> and
<code class="computeroutput">gthread</code> libraries. These libraries
are built on top of POSIX threads, and hence are directly supported by
DRD. Please keep in mind that you have to call
<code class="function">g_thread_init</code> before creating any threads, or
DRD will report several data races on glib functions. See also the
<a class="ulink" href="http://library.gnome.org/devel/glib/stable/glib-Threads.html" target="_top">GLib
Reference Manual</a> for more information about
<code class="function">g_thread_init</code>.
</p>
<p>
One of the many facilities provided by the <code class="literal">glib</code>
library is a block allocator, called <code class="literal">g_slice</code>. You
have to disable this block allocator when using DRD by adding the
following to the shell environment variables:
<code class="literal">G_SLICE=always-malloc</code>. See also the <a class="ulink" href="http://library.gnome.org/devel/glib/stable/glib-Memory-Slices.html" target="_top">GLib
Reference Manual</a> for more information.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.boost.thread"></a>8.2.8. Debugging Boost.Thread Programs</h3></div></div></div>
<p>
The Boost.Thread library is the threading library included with the
cross-platform Boost Libraries. This threading library is an early
implementation of the upcoming C++0x threading library.
</p>
<p>
Applications that use the Boost.Thread library should run fine under DRD.
</p>
<p>
More information about Boost.Thread can be found here:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      Anthony Williams, <a class="ulink" href="http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html" target="_top">Boost.Thread</a>
      Library Documentation, Boost website, 2007.
    </p></li>
<li class="listitem"><p>
      Anthony Williams, <a class="ulink" href="http://www.ddj.com/cpp/211600441" target="_top">What's New in Boost
      Threads?</a>, Recent changes to the Boost Thread library,
      Dr. Dobbs Magazine, October 2008.
    </p></li>
</ul></div>
<p>
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.openmp"></a>8.2.9. Debugging OpenMP Programs</h3></div></div></div>
<p>
OpenMP stands for <span class="emphasis"><em>Open Multi-Processing</em></span>. The OpenMP
standard consists of a set of compiler directives for C, C++ and Fortran
programs that allows a compiler to transform a sequential program into a
parallel program. OpenMP is well suited for HPC applications and allows one to
work at a higher level compared to direct use of the POSIX threads API. While
OpenMP ensures that the POSIX API is used correctly, OpenMP programs can still
contain data races. So it definitely makes sense to verify OpenMP programs
with a thread checking tool.
</p>
<p>
DRD supports OpenMP shared-memory programs generated by GCC. GCC
supports OpenMP since version 4.2.0.  GCC's runtime support
for OpenMP programs is provided by a library called
<code class="literal">libgomp</code>. The synchronization primitives implemented
in this library use Linux' futex system call directly, unless the
library has been configured with the
<code class="literal">--disable-linux-futex</code> option. DRD only supports
libgomp libraries that have been configured with this option and in
which symbol information is present. For most Linux distributions this
means that you will have to recompile GCC. See also the script
<code class="literal">drd/scripts/download-and-build-gcc</code> in the
Valgrind source tree for an example of how to compile GCC. You will
also have to make sure that the newly compiled
<code class="literal">libgomp.so</code> library is loaded when OpenMP programs
are started. This is possible by adding a line similar to the
following to your shell startup script:
</p>
<pre class="programlisting">
export LD_LIBRARY_PATH=~/gcc-4.4.0/lib64:~/gcc-4.4.0/lib:
</pre>
<p>
As an example, the test OpenMP test program
<code class="literal">drd/tests/omp_matinv</code> triggers a data race
when the option -r has been specified on the command line. The data
race is triggered by the following code:
</p>
<pre class="programlisting">
#pragma omp parallel for private(j)
for (j = 0; j &lt; rows; j++)
{
  if (i != j)
  {
    const elem_t factor = a[j * cols + i];
    for (k = 0; k &lt; cols; k++)
    {
      a[j * cols + k] -= a[i * cols + k] * factor;
    }
  }
}
</pre>
<p>
The above code is racy because the variable <code class="literal">k</code> has
not been declared private. DRD will print the following error message
for the above code:
</p>
<pre class="programlisting">
$ valgrind --tool=drd --check-stack-var=yes --read-var-info=yes drd/tests/omp_matinv 3 -t 2 -r
...
Conflicting store by thread 1/1 at 0x7fefffbc4 size 4
   at 0x4014A0: gj.omp_fn.0 (omp_matinv.c:203)
   by 0x401211: gj (omp_matinv.c:159)
   by 0x40166A: invert_matrix (omp_matinv.c:238)
   by 0x4019B4: main (omp_matinv.c:316)
Location 0x7fefffbc4 is 0 bytes inside local var "k"
declared at omp_matinv.c:160, in frame #0 of thread 1
...
</pre>
<p>
In the above output the function name <code class="function">gj.omp_fn.0</code>
has been generated by GCC from the function name
<code class="function">gj</code>. The allocation context information shows that the
data race has been caused by modifying the variable <code class="literal">k</code>.
</p>
<p>
Note: for GCC versions before 4.4.0, no allocation context information is
shown. With these GCC versions the most usable information in the above output
is the source file name and the line number where the data race has been
detected (<code class="literal">omp_matinv.c:203</code>).
</p>
<p>
For more information about OpenMP, see also 
<a class="ulink" href="http://openmp.org/" target="_top">openmp.org</a>.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.cust-mem-alloc"></a>8.2.10. DRD and Custom Memory Allocators</h3></div></div></div>
<p>
DRD tracks all memory allocation events that happen via the
standard memory allocation and deallocation functions
(<code class="function">malloc</code>, <code class="function">free</code>,
<code class="function">new</code> and <code class="function">delete</code>), via entry
and exit of stack frames or that have been annotated with Valgrind's
memory pool client requests. DRD uses memory allocation and deallocation
information for two purposes:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      To know where the scope ends of POSIX objects that have not been
      destroyed explicitly. It is e.g. not required by the POSIX
      threads standard to call
      <code class="function">pthread_mutex_destroy</code> before freeing the
      memory in which a mutex object resides.
    </p></li>
<li class="listitem"><p>
      To know where the scope of variables ends. If e.g. heap memory
      has been used by one thread, that thread frees that memory, and
      another thread allocates and starts using that memory, no data
      races must be reported for that memory.
    </p></li>
</ul></div>
<p>
</p>
<p>
It is essential for correct operation of DRD that the tool knows about
memory allocation and deallocation events. When analyzing a client program
with DRD that uses a custom memory allocator, either instrument the custom
memory allocator with the <code class="literal">VALGRIND_MALLOCLIKE_BLOCK</code>
and <code class="literal">VALGRIND_FREELIKE_BLOCK</code> macros or disable the
custom memory allocator.
</p>
<p>
As an example, the GNU libstdc++ library can be configured
to use standard memory allocation functions instead of memory pools by
setting the environment variable
<code class="literal">GLIBCXX_FORCE_NEW</code>. For more information, see also
the <a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt04ch11.html" target="_top">libstdc++
manual</a>.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.drd-versus-memcheck"></a>8.2.11. DRD Versus Memcheck</h3></div></div></div>
<p>
It is essential for correct operation of DRD that there are no memory
errors such as dangling pointers in the client program. Which means that
it is a good idea to make sure that your program is Memcheck-clean
before you analyze it with DRD. It is possible however that some of
the Memcheck reports are caused by data races. In this case it makes
sense to run DRD before Memcheck.
</p>
<p>
So which tool should be run first? In case both DRD and Memcheck
complain about a program, a possible approach is to run both tools
alternatingly and to fix as many errors as possible after each run of
each tool until none of the two tools prints any more error messages.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.resource-requirements"></a>8.2.12. Resource Requirements</h3></div></div></div>
<p>
The requirements of DRD with regard to heap and stack memory and the
effect on the execution time of client programs are as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      When running a program under DRD with default DRD options,
      between 1.1 and 3.6 times more memory will be needed compared to
      a native run of the client program. More memory will be needed
      if loading debug information has been enabled
      (<code class="literal">--read-var-info=yes</code>).
    </p></li>
<li class="listitem"><p>
      DRD allocates some of its temporary data structures on the stack
      of the client program threads. This amount of data is limited to
      1 - 2 KB. Make sure that thread stacks are sufficiently large.
    </p></li>
<li class="listitem"><p>
      Most applications will run between 20 and 50 times slower under
      DRD than a native single-threaded run. The slowdown will be most
      noticeable for applications which perform frequent mutex lock /
      unlock operations.
    </p></li>
</ul></div>
<p>
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.effective-use"></a>8.2.13. Hints and Tips for Effective Use of DRD</h3></div></div></div>
<p>
The following information may be helpful when using DRD:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      Make sure that debug information is present in the executable
      being analyzed, such that DRD can print function name and line
      number information in stack traces. Most compilers can be told
      to include debug information via compiler option
      <code class="option">-g</code>.
    </p></li>
<li class="listitem"><p>
      Compile with option <code class="option">-O1</code> instead of
      <code class="option">-O0</code>. This will reduce the amount of generated
      code, may reduce the amount of debug info and will speed up
      DRD's processing of the client program. For more information,
      see also <a class="xref" href="manual-core.html#manual-core.started" title="2.2. Getting started">Getting started</a>.
    </p></li>
<li class="listitem"><p>
      If DRD reports any errors on libraries that are part of your
      Linux distribution like e.g. <code class="literal">libc.so</code> or
      <code class="literal">libstdc++.so</code>, installing the debug packages
      for these libraries will make the output of DRD a lot more
      detailed.
    </p></li>
<li class="listitem">
<p>
      When using C++, do not send output from more than one thread to
      <code class="literal">std::cout</code>. Doing so would not only
      generate multiple data race reports, it could also result in
      output from several threads getting mixed up.  Either use
      <code class="function">printf</code> or do the following:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem"><p>Derive a class from <code class="literal">std::ostreambuf</code>
          and let that class send output line by line to
          <code class="literal">stdout</code>. This will avoid that individual
          lines of text produced by different threads get mixed
          up.</p></li>
<li class="listitem"><p>Create one instance of <code class="literal">std::ostream</code>
          for each thread. This makes stream formatting settings
          thread-local. Pass a per-thread instance of the class
          derived from <code class="literal">std::ostreambuf</code> to the
          constructor of each instance. </p></li>
<li class="listitem"><p>Let each thread send its output to its own instance of
          <code class="literal">std::ostream</code> instead of
          <code class="literal">std::cout</code>.</p></li>
</ol></div>
<p>
    </p>
</li>
</ul></div>
<p>
</p>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="drd-manual.Pthreads"></a>8.3. Using the POSIX Threads API Effectively</h2></div></div></div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.mutex-types"></a>8.3.1. Mutex types</h3></div></div></div>
<p>
The Single UNIX Specification version two defines the following four
mutex types (see also the documentation of <a class="ulink" href="http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutexattr_settype.html" target="_top"><code class="function">pthread_mutexattr_settype</code></a>):
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      <span class="emphasis"><em>normal</em></span>, which means that no error checking
      is performed, and that the mutex is non-recursive.
    </p></li>
<li class="listitem"><p>
      <span class="emphasis"><em>error checking</em></span>, which means that the mutex
      is non-recursive and that error checking is performed.
    </p></li>
<li class="listitem"><p>
      <span class="emphasis"><em>recursive</em></span>, which means that a mutex may be
      locked recursively.
    </p></li>
<li class="listitem"><p>
      <span class="emphasis"><em>default</em></span>, which means that error checking
      behavior is undefined, and that the behavior for recursive
      locking is also undefined. Or: portable code must neither
      trigger error conditions through the Pthreads API nor attempt to
      lock a mutex of default type recursively.
    </p></li>
</ul></div>
<p>
</p>
<p>
In complex applications it is not always clear from beforehand which
mutex will be locked recursively and which mutex will not be locked
recursively. Attempts lock a non-recursive mutex recursively will
result in race conditions that are very hard to find without a thread
checking tool. So either use the error checking mutex type and
consistently check the return value of Pthread API mutex calls, or use
the recursive mutex type.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.condvar"></a>8.3.2. Condition variables</h3></div></div></div>
<p>
A condition variable allows one thread to wake up one or more other
threads. Condition variables are often used to notify one or more
threads about state changes of shared data. Unfortunately it is very
easy to introduce race conditions by using condition variables as the
only means of state information propagation. A better approach is to
let threads poll for changes of a state variable that is protected by
a mutex, and to use condition variables only as a thread wakeup
mechanism. See also the source file
<code class="computeroutput">drd/tests/monitor_example.cpp</code> for an
example of how to implement this concept in C++. The monitor concept
used in this example is a well known and very useful concept -- see
also Wikipedia for more information about the <a class="ulink" href="http://en.wikipedia.org/wiki/Monitor_(synchronization)" target="_top">monitor</a>
concept.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="drd-manual.pctw"></a>8.3.3. pthread_cond_timedwait and timeouts</h3></div></div></div>
<p>
Historically the function
<code class="function">pthread_cond_timedwait</code> only allowed the
specification of an absolute timeout, that is a timeout independent of
the time when this function was called. However, almost every call to
this function expresses a relative timeout. This typically happens by
passing the sum of
<code class="computeroutput">clock_gettime(CLOCK_REALTIME)</code> and a
relative timeout as the third argument. This approach is incorrect
since forward or backward clock adjustments by e.g. ntpd will affect
the timeout. A more reliable approach is as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      When initializing a condition variable through
      <code class="function">pthread_cond_init</code>, specify that the timeout of
      <code class="function">pthread_cond_timedwait</code> will use the clock
      <code class="literal">CLOCK_MONOTONIC</code> instead of
      <code class="literal">CLOCK_REALTIME</code>. You can do this via
      <code class="computeroutput">pthread_condattr_setclock(...,
      CLOCK_MONOTONIC)</code>.
    </p></li>
<li class="listitem"><p>
      When calling <code class="function">pthread_cond_timedwait</code>, pass
      the sum of
      <code class="computeroutput">clock_gettime(CLOCK_MONOTONIC)</code>
      and a relative timeout as the third argument.
    </p></li>
</ul></div>
<p>
See also
<code class="computeroutput">drd/tests/monitor_example.cpp</code> for an
example.
</p>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="drd-manual.limitations"></a>8.4. Limitations</h2></div></div></div>
<p>DRD currently has the following limitations:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
      DRD, just like Memcheck, will refuse to start on Linux
      distributions where all symbol information has been removed from
      <code class="filename">ld.so</code>. This is e.g. the case for the PPC editions
      of openSUSE and Gentoo. You will have to install the glibc debuginfo
      package on these platforms before you can use DRD. See also openSUSE
      bug <a class="ulink" href="http://bugzilla.novell.com/show_bug.cgi?id=396197" target="_top">
      396197</a> and Gentoo bug <a class="ulink" href="http://bugs.gentoo.org/214065" target="_top">214065</a>.
    </p></li>
<li class="listitem"><p>
      With gcc 4.4.3 and before, DRD may report data races on the C++
      class <code class="literal">std::string</code> in a multithreaded program. This is
      a know <code class="literal">libstdc++</code> issue -- see also GCC bug
      <a class="ulink" href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40518" target="_top">40518</a>
      for more information.
    </p></li>
<li class="listitem"><p>
      If you compile the DRD source code yourself, you need GCC 3.0 or
      later. GCC 2.95 is not supported.
    </p></li>
<li class="listitem"><p>
      Of the two POSIX threads implementations for Linux, only the
      NPTL (Native POSIX Thread Library) is supported. The older
      LinuxThreads library is not supported.
    </p></li>
</ul></div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="drd-manual.feedback"></a>8.5. Feedback</h2></div></div></div>
<p>
If you have any comments, suggestions, feedback or bug reports about
DRD, feel free to either post a message on the Valgrind users mailing
list or to file a bug report. See also <a class="ulink" href="http://www.valgrind.org/" target="_top">http://www.valgrind.org/</a> for more information.
</p>
</div>
</div>
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